1. Field of the Invention
This invention relates to a method for preparing magnetic recording media such as magnetic tapes and magnetic discs.
2. Prior Art
For the manufacture of magnetic recording media having a coated magnetic layer, it is known to prepare the magnetic paint used therein by using a twin-shaft continuous kneader for applying high shear forces to a mixture of a magnetic powder, binder and organic solvent at a high solids concentration for dispersing the mixture.
As compared with the use of a batchwise kneader, for example, the use of a twin-shaft continuous kneader for the preparation of a magnetic paint offers high productivity, but a short kneading time because of continuous kneading operation. As a result, there is a frequent likelihood that kneading become insufficient and the dispersion of magnetic powder become poor, failing to provide satisfactory electromagnetic properties.
Then Japanese Patent Application: Kokai (JP-A) No. 176020/1992 proposes to knead a mixture at a temperature which is higher than the glass transition temperature (Tg) of the binder used and lower than the boiling point of the organic solvent used. This proposal ensures improved dispersion of magnetic powder and satisfactory electromagnetic properties, but is difficult to control the surface roughness of a magnetic recording medium prepared therefrom, with an improvement in surface profile entailing an increased coefficient of dynamic friction, often inviting a failure such as an accidental transport interruption. Although it is envisaged to reduce such conditions as the pressure and temperature of super-calendering as means for controlling surface roughness, undesirably this can often incur losses of high-frequency range output and C/N.